This invention relates generally to glow lamps constructed to have a relatively low breakdown voltage but a relatively high current-handling capability. This invention also relates generally to a method of manufacturing the same. More particularly, but not by way of limitation, this invention relates to a three-electrode glow lamp which has two sets of electrodes with each of the sets having one common electrode and which functions as a switch so that when a triggering breakdown voltage is applied to either of the sets of electrodes, a relatively large current flows between the two electrodes which are not common to both sets.
It is well known that glow lamps having two electrodes can be constructed and used as indicators or for other suitable uses. Three-electrode glow lamps are also known to the art, but these prior known lamps have been constructed so that only relatively small currents can be conducted therethrough. Such three-electrode glow lamps can be used as switches; however, the switching uses are limited because of the low current-handling capability.
It is also known to the art how to manufacture such two- and three-electrode glow lamps. Each of the electrodes is made of nickel-coated iron or other suitable substance on which a coating of barium carbonate (BaCO.sub.3) is applied. The appropriate number of electrodes is then placed in a housing, and the combined structure is placed in an induction heater where the barium carbonate decomposes into molecules of carbon dioxide (CO.sub.2) and barium oxide (BaO). The carbon dioxide is in a gaseous state and is pumped out of the housing, thereby leaving barium oxide on the surface of the electrodes. The lamp is then filled with a suitable gas, such as neon, and is placed in a sparking unit which applies high frequency voltages to the electrodes whereby high frequency currents flow between the electrodes so that the barium oxide decomposes into barium (Ba), which remains on the electrode, and molecules of ozone (O.sub.3). This voltage application step is continued for an appropriate time, such as approximately sixty seconds, until the electrodes can no longer conduct relatively large currents therebetween. At such time a solid barium film is left on the electrodes. It is also known that strontium carbonate (SrCO.sub.3) can be used with the barium carbonate in the foregoing process.
Barium (or strontium or other suitable material) is used because it is a good electron-emissive material which gives a relatively low breakdown voltage to the electrodes as compared to the nickel-coated iron electrodes alone. Although the performance of this prior art method results in a glow lamp which has a relatively low breakdown voltage, it is unable to pass relatively large currents when a breakdown voltage is applied. PG,4
Because prior known glow lamps have such a limited current-handling capability, there is the need for a glow lamp which has a relatively low breakdown voltage but which also is capable of conducting a relatively large current so that the glow lamp can be used, for example, as a switch in relatively large current-conducting circuits. Additionally, there is the need for a method of constructing a glow lamp so that it has these features of relatively low breakdown voltage, but relatively high current-handling capability.